Patent GB-1190804-A (GB)
Title: Improvements in or relating to Carrier Signal Transmission Systems
Inventor: HOCHGRAF LESTER
IPC: H04B1/74
Filed: 1967-06-14
Abstract: 1,190,804. Communication systems. WESTERN ELECTRIC CO. Inc. 14 June, 1967 [21 June, 1966], No. 27439/67. Heading H4R. In a multiplex communication system in which an office terminal 10, Fig. 1, can communicate, over appropriate time or frequency division multiplex channels with any of a number of remote terminals 11 to 14 connected in series on an outgoing line, and in which communication from any remote terminal to the office terminal takes place via the remaining remote terminals in the outward line and an inward line, loop back circuits 15, 16, and 17 are provided to respond to a break in the transmission path beyond any remote terminal to loop back the outward line to the inward line to maintain communication between the office terminal 10 and as many remote terminals as is possible, up to the point of the break in the transmission path. Operation of the loop back circuit may be arranged to put a characteristic signal in a supervisory channel of the system to indicate the identity of the looped-back circuit and therefore the location of the fault. In addition a command signal may be transmitted over a respective channel to cause operation of the loop back circuit for testing. The loop back circuit may be arranged to detect valid signals on the incoming side of the inward line and in the absence of such signals to disconnect the outgoing side of the inward line from the incoming side and connect it instead to the outward line, this arrangement ensuring that if the normal transmission path is restored, so that valid signals reappear on the incoming side of the inward line, the loop back condition is removed, Fig. 2 (not shown). Fig. 5 shows an arrangement for a loop back circuit associated with a remote terminal 60 in which absence of a valid signal on the outgoing side of the inward line causes operation of a relay 63 to connect the outgoing side of the terminal 60 to the outgoing side of the inward line instead of the outward line. Since the operation of the loopback circuit would restore valid signals on the valid signal detector, a logic circuit, Fig. 6 (not shown), is provided which allows restoration of the normal conditions if the valid signal is maintained, due to a loop back condition at a subsequent remote terminal, but restores the loop back condition if the valid signal ceases for longer than a predetermined time on release of the loop back condition. Instead of monitoring for valid signals the arrangement may be used for systems in which the remote terminals are supplied in series with power over the line and the loop back circuit may be arranged to complete a phantom circuit supplying power to the terminals on the detection of an interruption in the power supply current in the inward line, Fig. 7 (not shown). A valid signal detector for a time division system using bipolar pulses, i.e. in which message pulses are alternately positive and negative, is shown in Fig. 3. In this detector anti-phase signals are fed from transformer 40 to rectifiers 41 and 42 to produce two pulse trains of the same polarity, the output from rectifier 41 corresponding to positive pulses in the transmission and that from 42 corresponding to negative. The pulse trains are applied direct to one input of respective AND gates 43 and 44 and also via delays 45 and 46 and bi-stable 47 to the other inputs of the respective gates. The circuit is such that an output from the " AND " gates is only produced if the input signal is not bipolar, and the resulting output is fed via " OR " gate 49 to set monostable circuit 48, which is maintained as long as non-bipolar pulses continue to be received, to cause a loop back condition to be set up via drive circuit 50.